I could doubt you could doubt she could doubt we could doubt you could doubt they could doubt
V-ing form
I could be doubting you could be doubting she could be doubting we could be doubting you could be doubting they could be doubting
Perfect
I could have doubted you could have doubted she could have doubted we could have doubted you could have doubted they could have doubted
V-ing perfect
I could have been doubting you could have been doubting she could have been doubting we could have been doubting you could have been doubting they could have been doubting
Negation
Simple form
I couldn't doubt you couldn't doubt she couldn't doubt we couldn't doubt you couldn't doubt they couldn't doubt
V-ing form
I couldn't be doubting you couldn't be doubting she couldn't be doubting we couldn't be doubting you couldn't be doubting they couldn't be doubting
Perfect
I couldn't have doubted you couldn't have doubted she couldn't have doubted we couldn't have doubted you couldn't have doubted they couldn't have doubted
V-ing perfect
I couldn't have been doubting you couldn't have been doubting she couldn't have been doubting we couldn't have been doubting you couldn't have been doubting they couldn't have been doubting
Interrogative form
Simple form
could I doubt? could you doubt? could she doubt? could we doubt? could you doubt? could they doubt?
V-ing form
could I be doubting? could you be doubting? could she be doubting? could we be doubting? could you be doubting? could they be doubting?
Perfect
could I have doubted? could you have doubted? could she have doubted? could we have doubted? could you have doubted? could they have doubted?
V-ing perfect
could I have been doubting? could you have been doubting? could she have been doubting? could we have been doubting? could you have been doubting? could they have been doubting?
Interro-negative form
Simple form
couldn't I doubt? couldn't you doubt? couldn't she doubt? couldn't we doubt? couldn't you doubt? couldn't they doubt?
V-ing form
couldn't I be doubting? couldn't you be doubting? couldn't she be doubting? couldn't we be doubting? couldn't you be doubting? couldn't they be doubting?
Perfect
couldn't I have doubted? couldn't you have doubted? couldn't she have doubted? couldn't we have doubted? couldn't you have doubted? couldn't they have doubted?
V-ing perfect
couldn't I have been doubting? couldn't you have been doubting? couldn't she have been doubting? couldn't we have been doubting? couldn't you have been doubting? couldn't they have been doubting?